Adds Extension Store functionality to Xed Editor.

BREAKING CHANGE:

Extensions must now be installed via a .zip file containing both a plugin.json (with required metadata) and an .apk:

• plugin.json file with the following required fields:

  {
    "id": "<extension package name>", // REQUIRED
    "name": "<extension name>", // REQUIRED
    "mainClass": "<main class name>", // REQUIRED
    "version": "1.0.0",
    "description": "",
    "authors": [],
    "minAppVersion": -1,
    "targetAppVersion": -1,
    "repository": "",
    "tags": []
  }

• The .apk file for the extension.

Currently needs to implement:

• Enable/Disable plugin
• Reload when uninstalled
• Show install progress